The present invention relates to a method for producing iron carbide (Fe3C) suitable for raw materials for iron making or steel making which comprises iron carbide (Fe3C) as a main component, for example, raw materials for steel making which is used in an electric furnace and the like.
The production of steel normally comprises the steps of converting iron ore into pig iron using a blast furnace, and then converting the pig iron into steel using an open hearth furnace or a converter. Such a traditional method requires large amounts of energy and large-scale equipment, and has a high cost. Therefore, for a small-scale steel making, a method comprising the steps of directly converting iron ore into raw materials to be used in a steel making furnace, and converting the raw materials into steel using an electric furnace and the like has been used. With respect to this direct steel making process, a direct reduction process has been used to convert iron ore into reduced iron. However, the reduced iron produced by the direct reduction process is highly reactive and reacts with oxygen in the air to generate heat. Therefore, it is necessary to seal the reduced iron with an inert gas or by some other measures during transportation and storage of the reduced iron. Accordingly, iron carbide (Fe3C) containing a comparatively high iron (Fe) content, and which has a low reaction activity and can be easily transported and stored, has recently been used as the raw materials for steel making in an electric furnace and the like.
Furthermore, raw materials for iron making or steel making containing iron carbide as a main component is not only easy to be transported and stored, but also has the advantage that carbon element combined with iron element can be used as a source of fuel in an iron making or steel making furnace, and can be used as a source to generate microbubbles which accelerates a reaction in the steel making furnace. Therefore, raw materials for iron making or steel making containing iron carbide as a main component has recently attracted special interest.
According to a conventional method for producing iron carbide, iron ore fines are fed into a fluidized bed reactor or the like, and are caused to react with a gas mixture comprising a reducing gas (e.g., hydrogen gas) and a carburizing gas (e.g., a methane gas and the like) at a predetermined temperature. Thus, iron oxides (hematite (Fe2O3), magnetite (Fe3O4), wustite (FeO)) contained in iron ore are reduced and carburized in a single process (which means a process performed by simultaneously introducing a reducing gas and a carburizing gas into a single reactor) as shown in the following reaction formula ((1), (2), (3), (4)).
3Fe2O3+H2xe2x86x922Fe3O4+H2Oxe2x80x83xe2x80x83(1) 
Fe3O4+H2xe2x86x923FeO+H2Oxe2x80x83xe2x80x83(2) 
FeO+H2xe2x86x92Fe+H2Oxe2x80x83xe2x80x83(3) 
3Fe+CH4xe2x86x92Fe3C+2H2xe2x80x83xe2x80x83(4) 
Furthermore, the formula (1) to (3) may be put together. As a result, the reaction formula for deciding the progress of reduction and carburization of iron oxide can be shown by the following reaction formula (5) and the above-mentioned reaction formula (4).
Fe2O3+3H2xe2x86x922Fe+3H2Oxe2x80x83xe2x80x83(5) 
The prior art related to the field of the present invention has been described in the publication No. 6-501983 of Japanese Translation of International Patent Application (PCT/US91/05198), for example.
However, free carbon is sometimes generated depending on the producing conditions such as a gas composition, a reaction temperature and the like in a fluidized bed reactor. In a case where the free carbon is mixed with iron carbide, there come out the following drawbacks.
(1) There is a possibility that a dust fire or explosion might be caused by free carbon scattered in a gas exhausted from the fluidized bed reactor.
(2) An opening such as an inlet port for raw materials, an outlet port for product of the fluidized bed reactor is sometimes blocked by the free carbon.
(3) CH4 is consumed with the generation of the free carbon. For this reason, CH4 which is necessary for the generation of iron carbide is additionally required.
(4) Once the free carbon is generated, the generation of the free carbon is promoted.
In consideration of the above-mentioned problems of the prior art, it is an object of the present invention to provide a method for producing iron carbide in which free carbon is generated with difficulty.
In order to accomplish the above-mentioned object, the present invention can detect free carbon in a latter half of compartments in a reactor and change a composition of a reaction gas corresponding to a quantity of the detected free carbon, thereby controlling the generation of the free carbon.
More specifically, the present invention provides a method for preventing free carbon from being generated by thermal decomposition from carbon monoxide or hydrocarbon when raw materials for iron making or steel making mainly comprising iron carbide as a main component is produced by reducing and carburizing iron-containing raw materials for iron making mainly comprising iron oxides and iron hydroxides as main components using a reaction gas mainly containing hydrogen and methane, comprising the steps of dividing an inside of a reactor into a plurality of compartments, detecting the free carbon in a latter half of the compartments which are closer to an outlet port for product, and changing a composition of the reaction gas corresponding to a quantity of the detected free carbon.
As a process of detecting free carbon, it is possible to employ xe2x80x9ca process of detecting free carbon based on a change of a temperature of a thermometer installed in the latter half of the compartmentsxe2x80x9d, xe2x80x9ca process of detecting free carbon by analyzing dust picked up by a dust pick-up device installed on an upper portion in the latter half of the compartmentsxe2x80x9d or xe2x80x9ca process of detecting free carbon based on a result of detection of a ratio of methane to hydrogen in gas picked up by a gas pick-up device installed on an upper portion in the latter half of the compartmentsxe2x80x9d. In addition, these two or more processes can be combined.
As a process of changing the composition of the reaction gas in the reactor, it is possible to employ xe2x80x9ca process of increasing steam in the reaction gas corresponding to the quantity of the detected free carbonxe2x80x9d, xe2x80x9ca process of increasing carbon dioxide in the reaction gas corresponding to the quantity of the detected free carbonxe2x80x9d or xe2x80x9ca process of decreasing the carbon monoxide or the hydrocarbon in the reaction gas corresponding to the quantity of the detected free carbonxe2x80x9d. In addition, these two or more processes can be combined.
The above-mentioned change of the composition of the reaction gas can be performed by controlling the composition of the reaction gas to be introduced into the reactor.
The status of the change of iron-containing raw materials in the reactor according to the present invention having the above-mentioned constitution will generally be described. A part of raw materials in the compartments on the side closer to the inlet port for raw materials is converted into Fe by mainly reducing reaction. Then, the residual reducing reaction and the carburizing reaction into iron carbide (Fe3C) of the iron-containing raw materials are performed in the compartments on the side closer to the outlet port for product. If the carburizing reaction in which an upper limit of a ratio of conversion into Fe3C obtained at a certain H2O partial pressure is exceeded is carried out, free carbon is generated. By the following process, the free carbon is detected. Then, the composition of the reaction gas is changed by the following process corresponding to the quantity of the detected free carbon. Consequently, the generation of the free carbon can be controlled.
(1) Detection of Free Carbon
a. Change of Reaction Temperature
The change of a reaction temperature can be used as means for indirectly detecting the generation of free carbon. The reason is as follows. It is supposed that a temperature in a reactor is generally uniform when reaction steadily progresses in a fluidized bed reactor. However, deposition of the free carbon which is caused in the reactor is an endothermic reaction. For this reason, a temperature is dropped in a portion where the free carbon is deposited. Therefore, thermometers are installed in a portion where the free carbon is easily deposited (a portion closer to the outlet port for product of the reactor) and other portions in the reactor where the free carbon is deposited with difficulty, and their measuring temperatures are compared with one another. If it is apparent that the temperature in the portion closer to the outlet port for product is clearly lower than the temperatures in the other portions, it is possible to decide that the free carbon is deposited.
b. Analysis of Dust
By analyzing dust picked up by means of the dust pick-up device, the free carbon can be directly detected.
c. Analysis of Gas Composition
As described above, the free carbon is generated when the ratio of conversion of Fe into Fe3C has become equal to or greater than a constant value. In other words, if a ratio of CH4/H2 in the gas composition is raised to a constant value or more as shown by the above-mentioned reaction formula (4), there is a higher possibility that the free carbon might be generated. Accordingly, the gas composition in the latter half of the compartments of the reactor is analyzed, and the generation of the free carbon can be indirectly detected if a value of CH 4/H2 in the above gas has been rapidly reduced.
d. Combination of Detecting Process
If two or more of xe2x80x9cchange of reaction temperaturexe2x80x9d, xe2x80x9canalysis of dustxe2x80x9d and xe2x80x9canalysis of gas compositionxe2x80x9d are combined, the free carbon can be detected more rapidly.
(2) Change of Reaction Gas Composition
a. Addition of Steam (H2O)
It is not preferable that a ratio of conversion from iron-containing raw materials into iron carbide be low (equal to or lower than about 90%). The reason is that the grade of the iron carbide having the low conversion ratio is too low to be used as raw materials for iron making or steel making. On the other hand, if the ratio of conversion from iron-containing raw materials into iron carbide is too high (equal to or higher than about 99%), free carbon is easily deposited. Therefore, if it is hoped that raw materials for iron making or steel making with high grade is obtained while controlling the generation of the free carbon, it is preferable that the ratio of conversion from iron-containing raw materials into iron carbide should be kept within a constant range.
As described above, it is possible to collectively express, by the following two reaction formula, the reaction where iron carbide (Fe3C) is produced from iron-containing raw materials using a reaction gas mainly containing hydrogen and methane.
Fe2O3+3H2xe2x86x922Fe+3H2Oxe2x80x83xe2x80x83(5) 
3Fe+CH4xe2x86x92Fe3C+2H2xe2x80x83xe2x80x83(4) 
In the last stage of the conversion into Fe3C (in which most of the iron-containing raw materials are converted into Fe3C), the reaction approaches an equilibrium state. If steam (H2O) is added into a reaction system in the equilibrium state, potentials of hydrogen and oxygen are increased and potentials of iron and carbon are decreased. More specifically, when H2O is added to the reaction system of the reaction formula (5), a concentration or partial pressure of a molecule on the right side is raised. For this reason, the reaction progresses toward the left side to recover the equilibrium state. In other words, H2 is increased and Fe is decreased. In the reaction formula (4), the reaction progresses toward the left side in order to consume the increase in H2 and to compensate for the decrease in Fe. As a result, it is possible to control excessive conversion from iron-containing raw materials into iron carbide.
Thus, by adding the steam to the reaction gas, the generation of the free carbon can be controlled. In addition, it is possible to obtain raw materials for iron making or steel making with high grade in which a ratio of conversion into iron carbide is kept within a proper range.
b. Addition of Carbon Dioxide (CO2)
In a case where carbon dioxide is added in place of steam, the following reaction progresses.
CO2+H2xe2x86x92CO+H2Oxe2x80x83xe2x80x83(6) 
More specifically, addition of CO2 is synonymous with that of H2O. By the above-mentioned action, the generation of free carbon can be controlled.
c. Decrease in Quantity of Methane (CH4) to be Supplied
A decrease in the quantity of CH4 to be supplied is equivalent to the progress of the reaction in the reaction formula (4) toward the left side or a delay of the progress of the reaction in the reaction formula (4) toward the right side. As a result, excessive conversion from iron-containing raw materials into iron carbide can be controlled. Consequently, the generation of the free carbon can be controlled. If a quantity of CH4 to be supplied is excessively reduced, the generation of the free carbon can be controlled but a quantity of metallic iron (Mxe2x80x94Fe) accumulated in a product is increased. The metallic iron contained in the product gradually reacts with oxygen in the air at an ordinary temperature and is then returned to iron oxide. Accordingly, it is preferable that the quantity of CH4 to be supplied should not be reduced excessively but be reduced to obtain such a gas composition to approach an equilibrium state with a solid at that time. Referring to an equilibrium of the gas composition in Fexe2x80x94CHO reaction system, in other words, a concentration of CH4 is reduced to approach the equilibrium state in order to correct a carbon potential which is excessively higher than an equilibrium composition of Fe and Fe3C. Consequently, the generation of the free carbon can be controlled.
d. Combination of Processes of Changing Reaction Gas Composition
If two or more of xe2x80x9caddition of steamxe2x80x9d, xe2x80x9caddition of carbon dioxidexe2x80x9d and xe2x80x9cdecrease in quantity of methane to be suppliedxe2x80x9d are combined, the generation of free carbon can be controlled more effectively.
(3) Process of Changing Reaction Gas Composition
By controlling a flow rate of a gas to be exhausted from the reactor, a gas composition in the reactor can be changed. However, if this process is carried out, a gas balance within the reactor is sometimes lost. Therefore, this process is not preferred.
As compared with the foregoing, a process of increasing a steam content of the reaction gas to be introduced into the reactor, adding steam or carbon dioxide to the reaction gas or decreasing a quantity of methane to be supplied can be easily performed directly and has no drawbacks which are caused by the process of controlling the flow rate of the exhaust gas.
(4) Other Processes of Controlling Generation of Free Carbon
Other processes of controlling the generation of free carbon than the above-mentioned processes are as follows.
a. Increase in Quantity of Iron-Containing Material to be Fed and Increase in Quantity of Iron Carbide to be Discharged
If the quantity of iron carbide to be discharged is increased, iron carbide in which free carbon is actually deposited is discharged and iron-containing raw materials having a large number of nonreacted components is increased in the vicinity of the outlet port of the reactor. Once the free carbon is deposited, the generation of the free carbon is promoted. Therefore, it is preferable that the deposited free carbon should be discharged quickly. Even if the quantity of iron-containing raw materials to be fed is increased, the same action can be obtained.
b. Feeding Nonreacted Iron-Containing Raw Materials in Stage in Which Reducing Reaction and Carburizing Reaction have Progressed to Constant Extent or More
If nonreacted iron-containing raw materials are fed into a portion in which reducing reaction and carburizing reaction have progressed to a constant extent or more and a ratio of conversion into iron carbide is very high (for example, a portion in the vicinity of the outlet port of the reactor), oxygen potential is increased in that portion. Therefore, H2O is generated and excessive conversion into iron carbide is inhibited in the same manner as the addition of steam. Consequently, the generation of the free carbon can be controlled.
c. Drop in Reaction Temperature
If a temperature is dropped, a reaction speed is reduced so that the generation of the free carbon can be controlled.
(5) Means for Predicting Generation of Free Carbon
The quantity of metallic iron contained in a product can be used as means for predicting the generation of free carbon. The reason is that the ratio of the metallic iron contained in the product is low in the last stage of the reaction, and iron produced by reducing residual iron oxide immediately is liable to be converted into iron carbide. More specifically, very small quantity of metallic iron (or lack of the metallic iron) indicates excessive conversion into iron carbide. If a quantity of the metallic iron contained in the product is known, the generation of the free carbon can be predicted. A specific method for predicting the generation of the free carbon is as follows. A gas composition in the vicinity of the outlet port of the reactor (which has an equilibrium relationship with the product) is known by a gas chromatography method, for example, and the quantity of the metallic iron contained in the product is estimated using the gas composition as a basis. Thus, the generation of the free carbon can be predicted.
According to the present invention described above, the inside of the reactor is divided into a plurality of compartments, free carbon is detected in a latter half of the compartments which are closer to the outlet port for product, and the composition of a reaction gas is changed corresponding to the quantity of the detected free carbon. Thus, the generation of the free carbon can precisely be detected and the generation of the free carbon can be controlled.
By employing the above-mentioned process of detecting the free carbon, the generation of the free carbon can be detected more surely.
Furthermore, the above-mentioned process is employed to change the composition of the reaction gas in the reactor. Consequently, the generation of the free carbon can be controlled ffectively. Therefore, it is possible to eliminate drawbacks such as blocking of the reactor, a dust fire and the like which are caused by the free carbon. In addition, it is possible to produce iron carbide as raw materials for iron making or steel making with high grade to which no carbon sticks. Furthermore, since the generation of the free carbon can be controlled effectively, it is possible to prevent wasteful consumption of carbon monoxide or hydrocarbon which acts as a resource for producing iron carbide.